a) Field of the Invention
The invention is directed to a process for generating at least three light bundles of different wavelength, especially for displaying color images, one of these light bundles having a longest wavelength and another of the light bundles having a shortest wavelength, which light bundles are obtained in the process with an OPO and further nonlinear optical component elements, such as devices for generating higher harmonics and/or sum frequency mixers and/or difference frequency mixers, from a signal beam and/or idler beam of the OPO and/or from a primary light bundle, from which a beam exciting the OPO is also derived.
b) Description of the Related Art
The invention is further directed to an apparatus for generating at least three light bundles of different wavelength, especially for displaying color images, one of these light bundles having a longest wavelength and another of the light bundles having a shortest wavelength, with a laser for generating a primary light bundle, with an OPO from which a signal beam and/or an idler beam can be taken after excitation by means of a partial light bundle of the primary light bundle, and with a device having nonlinear optical elements, wherein a further light bundle, the idler beam and/or the signal beam, is introduced into this device, and wherein the light bundle with the longest wavelength and the light bundle with the shortest wavelength can be coupled out of this device.
The expression "partial light bundle" which is employed hereinafter is understood herein to mean not only a light bundle which is split off by means of a beam splitter, but also a light bundle which is formed in and exits from nonlinear crystals as an unconverted component of the exciting beam.
Devices of the type mentioned above are known, for example, from DE 195 04 047 C1 and from WO 96/08116. Although this prior art is exclusively concerned with the generation of red, green and blue beams for use in a color video system, this technique can also be used in printing. In this connection, the wavelengths are not necessarily selected according to the color sensitivity of the human eye because, in such applications, the color selection of the laser is also substantially related to the sensitivity of the film to be exposed or the surface to be imprinted. Further, this technique is also not limited to three colors, that is, to three light bundles of different wavelength, since, at the present time, color pictures of high quality, for example art prints, are usually produced with more than three colors, in four-color or even six-color printing.
While the technique is also applicable in other areas such as the printing industry, reference is had herein essentially to laser video engineering as is known, for example, from DE 43 06 797 C1, in which only three laser beams with the colors red, green and blue are normally used. In an application of this kind, the light bundle with the shortest wavelength is blue and the light bundle with the longest wavelength is red.
In the present state of the art, difficulties are encountered particularly in generating the required shortest-wave blue light bundle using economical lasers with sufficiently high efficiency.
In DE 195 04 047 C1 and WO 96108116, it is suggested, in order to reduce expenditure for laser video projection, to obtain all of the light bundles with the different wavelengths for the colors red, green and blue from an individual infrared laser with the help of nonlinear optical elements. An optical parametric oscillator, hereinafter referred to as OPO, as is known, for example, from DE 42 19 169 A1, is used, above all, for this purpose. In an OPO of this type, an exciting light bundle is introduced into a nonlinear optical crystal. As a result of the optical nonlinearity, two additional beams of different frequency, the signal beam and the idler beam, can be obtained in addition to the exciting beam, depending on the orientation of the crystal and/or the temperature and/or the wavelength of the exciting beam.
According to the prior art indicated above, the signal beam and idler beam are then mixed together to form three light bundles with wavelengths suitable for laser projection by means of frequency summing or by forming higher harmonics via additional nonlinear optical elements. In particular, a partial light bundle of the green laser beam is used in addition for this purpose in order to effectively generate the blue light bundle and the red light bundle, that is, the light bundle with the shortest wavelength as well as the light bundle with the longest wavelength.
With regard to the special technique of the OPO, frequency mixing and the generation of higher harmonics, reference is had in particular to the prior art mentioned above and to the references cited therein.
WO 96/08116, above all, is useful for realizing this technique. Various possibilities are given therein for generating three light bundles of different wavelength with OPOs. This reference also includes a table for material selection for the OPO crystals, the wavelengths made possible by means of these materials for the signal beam and idler beam and physical parameters to be taken into consideration, required temperature regulating accuracy, operating temperature, and crystal cut or orientation and the like data which allow the person skilled in the art to build corresponding OPOs with the required regulating devices and heating means.
Examination of these tables shows, however, that extensive compromises must be made for a laser system for color display that is usable in practice. Either a high temperature accuracy is required, the crystals must sometimes also be operated at temperatures higher than room temperature, or the indicated possibilities are dependent on the use of light bundles with wavelengths in the far infrared range or in the ultraviolet range. In the far infrared range or UV range, only a slight transparency is to be expected in many of the indicated crystals; that is, the crystals partially absorb the laser energy, which adds to the difficulty of phase matching and accordingly of temperature stabilization for the required regulating accuracy and also results in a reduced intensity for the light bundles that are usable for video projection, i.e., lowers efficiency. As experiments have shown, the absorption can even cause an unforeseeable destruction of the crystals.
When crystals are used at an operating temperature above room temperature, an initial warmup period is necessary after switching on so that a sufficiently high stability for proper operation of a video system is possible only after a long period of time. Although this startup time could be reduced by means of higher heat output and improved regulation, this would required an increase in electronic apparatus.